The molecular properties of apolipoproteins have been shown to be quite sensitive to numerous experimental conditions, including protein concentration, solvent composition, pH, temperature and pressure. Apolipoproteins A-I, A-II, and C-I each self-associate in aqueous solution with major changes in secondary, tertiary and quaternary structure. In addition, the molecular properties of apoC-III have been found to depend strongly on carbohydrate content. It is inferred from these data that one of the control points in the molecular interactions and metabolism of plasma lipoproteins is the specific conformation of apolipoproteins in lipoproteins and lipoprotein particles. These and other experimental observations have been used to form a framework for the evaluation of lipid-lipid, lipid-protein and protein-protein interactions in lipoproteins and lipoprotein particles. A knowledge of these forces is fundamental to our understanding of lipid transport and metabolism in normal and abnormal states. BIBLIOGRAPHIC REFERENCES: Osborne, J.C. and Brewer, H.B., Jr.: The plasma lipoproteins. Advan. Protein Chem. 31: 253-337, 1977. Moss, J., Osborne, J.C., Fishman, P., Brewer, H.B., Jr., Vaughn, M., and Brady, R.O.: Effect of gangliosides and substrate analogues on the hydrolysis of nicotinamide adenine dinucleotide by choleragen. Proc. Natl. Acad. Sci. U.S.A., 74: 74-78, 1977.